UC3854N

The Texas Instruments UC3854N is a power factor correction controller that provides active power factor correction for off-line power supplies by shaping the input line current to match the input line voltage waveform. Using average current mode control with continuous conduction mode boost topology, the device achieves near-unity power factor. The UC3854 integrates a voltage error amplifier, current error amplifier, analog multiplier/divider with line voltage feedforward, PWM comparator, and oscillator. The internal 7.5V precision reference provides stable voltage regulation. Overvoltage protection, undervoltage lockout, and soft-start features ensure safe and reliable operation. The device is the industry-standard PFC controller widely used in medium to high power applications.

The UC3854N operates through three nested control loops: 1) Voltage Loop: The voltage error amplifier compares a fraction of the boost output voltage with the 7.5V internal reference, producing an error signal VEAO that represents the required power level. 2) Multiplier: The analog multiplier divides VEAO by the square of the average input voltage (feedforward), then multiplies by the instantaneous rectified line voltage. This produces a current reference signal with the correct sinusoidal shape and amplitude for near-unity power factor. 3) Current Loop: The current error amplifier compares the multiplier output with the actual inductor current sensed through a current shunt. The average current mode control ensures the inductor current accurately tracks the sinusoidal reference regardless of inductor ripple. The PWM comparator generates gate drive pulses to the external boost MOSFET.

• Off-line 300W-2000W SMPS PFC pre-regulator stage
• Server and telecom power supply with active PFC
• Industrial equipment meeting IEC 61000-3-2 harmonic limits